The instant invention relates to bimetallic disc actuated thermostatic switches and more particularly to a bimetallic disc actuated thermostatic switch which includes a thermal override protection and which is adapted for miniaturization.
The possibility of undesirable thermal override of a bimetallic disc actuated thermostatic switch is often of prime concern in electronic circuit design. In this connection, it will be recognized that when a thermostatic switch is used to control the temperature in a particular area or environment, the failure of the switch to interrupt continuity in circuitry at the proper temperature level may result in significant malfunction of or damage to mechanical and/or electrical components due to overheating. While normally bimetallic disc actuated thermostatic switches are highly reliable, the extensive malfunction or damage which can result from the failure thereof makes the possibility of such malfunction a matter of grave concern. It will be recognized that the malfunction of a thermostatic switch can result from disc malfunction, drift, contact sticking, creep resulting from contact wear, physical damage to the switch from external sources or a number of other conditions. However, regardless of the cause, thermostatic switch malfunction must be considered in many bimetallic disc actuated thermostatic switch applications.
It is seen, therefore, that in many instances there is a need to provide some type of thermal override protection in circuitry controlled by bimetallic disc actuated thermostatic switches. In some cases this need has been fulfilled by providing thermal fuse protection which interrupts the circuitry when excessive temperature conditions are present. Examples of thermal fusing devices which are usable in applications of this type are disclosed in the U.S. Pat. No. 4,307,370, to Hollweck, and Sackamoto et al., U.S. Pat. No. 4,065,741. The concept of providing thermal override protection within a thermostatic switch is also generally known and is disclosed in the European patent application No. 80300246.8 to Eaton, which represents the closest prior art to the instant invention of which the applicant is aware. However, while the broad concept of providing thermal override protection in a thermostatic switch is generally known, a means of providing thermal override protection for miniature thermostatic switches has not been heretofore available. Specifically, the structural components heretofore available for providing thermal override protection in thermostatic switches have not been readily adaptable to miniaturization and hence their applicability has been limited in this area.
The instant invention provides a simple and effective thermal override protection for a bimetallic disc actuated thermostatic switch which permits effective miniaturization of the switch. In this regard the instant invention relates to a thermal override which can be used in a switch of the type disclosed in the applicant's copending U.S. patent application Ser. No. 347,842, which is soon to issue as U.S. Pat. No. 4,367,452. The thermostatic switch of the instant invention comprises a base portion having a fulcrum thereon, a housing mounted on the base portion, a fixed contact mounted adjacent the base portion, and a movable contact arm which overlies the fulcrum. A movable contact is provided on one end of the movable arm and the arm is pivotable on the fulcrum to cause the engagement or disengagement of the fixed and movable contacts to effect or interrupt continuity therebetween. A bimetallic disc of the switch is mounted in the housing in communication with the movable contact arm and flexes when the temperature in the surrounding environment rises to a predetermined level to cause the movable arm to be pivoted on the fulcrum and thereby interrupt electrical continuity between the fixed and movable contacts. The switch further comprises a thermal override pellet which is mounted in a cavity in the base portion and which is deformable from a first undeformed position thereof to a second deformed position of reduced profile relative to the base portion in response to a predetermined override or excessive temperature. The fixed contact of the switch is mounted in communication with the pellet and is biased thereagainst and accordingly when the pellet is deformed, the fixed contact automatically moves to a position wherein it is disengaged from the movable contact even though the latter is in its normally engaged position. As a result, while continuity between the fixed and movable contacts is normally effected or interrupted through the flexing of the bimetallic disc, if the switch malfunctions and the temperature in the surrounding environment reaches an excessive level the override pellet is deformed causing the fixed contact to be moved away from the movable contact to interrupt continuity and thereby provide a thermal override protection in the switch.
It is, therefore, a primary object of the instant invention to provide a thermal override protection for a bimetallic disc actuated thermostatic switch.
Another object of the instant invention is to provide a bimetallic disc actuated thermostatic switch with a thermal override protection which is suitable for miniaturization.
A still further object of the instant invention is to provide a bimetallic disc actuated thermostatic switch wherein a fixed contact of the switch is permanently moved to an inoperative position to interrupt electrical continuity in the switch in response to overheating conditions.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawing.